I've never actually witnessed I've just seen the people who have been involved in accidents. It is worst when you get a person who hasn't been wearing their leathers so when they come off the bike you get this glove effect where areas of you skin, especially on your arms and legs, get ripped off like pulling off a glove.

Leathers are good because they stop that happening or if it does happen you can get the skin from inside the leathers instead of off the road. However they do not decrease impulse so if you go into something hard, it isn't going to help at all.